In "Grudge Match," De Niro and Stallone play Billy "The Kid" McDonnen and Henry "Razor" Sharp, two local Pittsburgh fighters whose fierce rivalry put them in the national spotlight. Each had scored a victory against the other during their heyday, but in 1983, on the eve of their decisive third match, Razor suddenly announced his retirement, refusing to explain why but effectively delivering a knock-out punch to both their careers. Thirty years later, boxing promoter Dante Slate Jr., seeing big dollar signs, makes them an offer they can't refuse: to re-enter the ring and settle the score once and for all.

But they may not have to wait that long: on their first encounter in decades, their long-festering feud erupts into an unintentionally hilarious melee that instantly goes viral. The sudden social media frenzy transforms their local grudge match into a must-see HBO event. Now, if they can just survive the training, they may actually live to fight again.